Self-organization of hydrogen-bonding naphthalene chromophores into J-type nanorings and H-type nanorods: Impact of regioisomerism

Article abstract of DOI:10.1002/anie.201201436

Regioisomers of self-assembling molecules resulted in the evolution of distinct 0D and 1D nanostructures, namely nanorings and nanorods (see picture). J- and H-type excitonic coupling of naphthalene chromophores was found in the nanostructures. The bulk l

Full text of DOI:10.1002/anie.201201436

Angewandte
Chemie
DOI: 10.1002/anie.201201436
Self-Assembly
Self-Organization of Hydrogen-Bonding Naphthalene Chromophores
into J-type Nanorings and H-type Nanorods: Impact of
Regioisomerism**
Shiki Yagai,* Yusaku Goto, Xu Lin, Takashi Karatsu, Akihide Kitamura, Daiki Kuzuhara,
Hiroko Yamada, Yoshihiro Kikkawa, Akinori Saeki, and Shu Seki
Nature has evolved complex self-organized architectures of
pigment assemblages with rational nanoscale topologies and
chromophore orientation.[1] Circular architectures of the
chlorophyll–protein complexes found in the light-harvesting
systems of purple photosynthetic bacteria could be regarded
as perfect supramolecular assemblies of pigments in view of
not only their topological features but also a functional
standpoint.[1b,2] Sunlight is absorbed uniformly by circularly
organized chlorophyll pigments, and the resulting exciton
states are delocalized over the closed arrays of chlorophylls.[3]
Such exciton delocalization is achieved by partially over-
lapped arrays of chlorophyll p systems, which could be
referred to as J-type (offset) stacking.[4] In contrast, one-
dimensional stacks of largely overlapped p systems, which can
be referred to as H-type (face-to-face) stacking, are promising
as quasi one-dimensional pathways of mobile charge carriers
for organic electronics.[5] Thus, these natural and artificial
assemblies of p systems show the importance of simultaneous
control over dimensionality of nanostructures and local
stacking arrangements to optimize the functionality in the
systems. Herein we report that a regioisomerism in specifi-
cally designed self-assembling small molecules can offer the
aforementioned two extreme nanoarchitectures with favor-
able stacking arrangements of their p systems.
We have recently reported the self-assembly of p-con-
jugated molecules substituted asymmetrically with barbituric
acid (BAR) and a wedge-shaped aliphatic tail (“miniden-
dron”[6]).[7] An unique feature of these BAR–p-wedge mol-
ecules in nonpolar media is that their ability to form
dramatically different nanostructures, namely nanorings[7a,b,8]
and nanorods,[7c] depending on the structure of the p moiety.
X-ray diffraction analysis in the mesomorphic state suggested
that the BAR–p-wedge molecules form columnar stacks of
hydrogen-bonded hexamers (rosettes), which are in nonpolar
media solvated to form curved (for nanorings) or straight
nanostructures (for nanorods). Although this molecular
design is attractive for the construction of well-defined
nanostructures consisting of functional p systems, it remains
difficult to clarify the structural relationship between mono-
mers and assemblies. We now report a striking impact of the
regioisomerism of BAR–naphthalene-wedge molecules
(Figure 1) on their self-assembled nanostructures, which
[*] Prof. Dr. S. Yagai, Y. Goto, Dr. X. Lin, Prof. Dr. T. Karatsu,
Prof. Dr. A. Kitamura
Department of Applied Chemistry and Biotechnology
Graduate School of Engineering, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
E-mail: yagai@faculty.chiba-u.jp
Dr. D. Kuzuhara, Prof. Dr. H. Yamada
Graduate School of Material Science
Nara Institute of Science and Technology (NAIST)
8916-5, Takayama-cho, Ikoma, Nara 630-0192 (Japan)
Prof. Dr. S. Yagai, Prof. Dr. H. Yamada
CREST, JST, 5-3, Yonbancho, Chiyoda-ku
Tokyo 102-8666 (Japan)
Dr. Y. Kikkawa
National Institute of Advanced Industrial Science and Technology
(AIST)
1-1-1 Higashi, Tsukuba, Ibaraki 305-8562 (Japan)
Figure 1. Structures of BAR–naphthalene-wedge molecules 1 and 2 and
their geometry-optimized structures.
Dr. A. Saeki, Prof. Dr. S. Seki
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1, Yamadaoka, Suita, Osaka 565-0871 (Japan)
imparts an important guide to this molecular design toward
tailor-made nanostructures. Introduction of BAR and wedge
functional moieties into different positions on the naphtha-
lene core affords the rosettes with different geometrical
features that are favorable for offset or face-to-face stacking
arrangements. This geometrical difference at the level of
rosettes not only yields perfect nanorings and nanorods, but
also enables favorable J- and H-type excitonic coupling of the
naphthalene cores, thus exhibiting distinct optical features.
[**] We are grateful to Prof. Tadashi Mizoguchi and Prof. Hitoshi
Tamiaki of Ritsumeikan University for the DLS measurements. This
work was partially supported by the Green Photonics Project in
NAIST sponsored by the Ministry of Education, Culture, Sports,
Science and Technology, MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201436.
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Regioisomers 1 (substituted at the 2,6-positions) and 2
(1,4-positions) were prepared according to the reported
method[7a] and fully characterized by NMR, MS, and elemen-
tal analysis.[9] The naphthalene rings in these regioisomers are
rotatable in the axis connecting their substituted positions.
Regioisomer 2, the naphthalene ring of which is substituted
on its short molecular axis, may have a greater steric repulsion
with neighboring molecules compared to 1 when they
assemble into rosette architectures. This may lead to a greater
rotational twisting of the naphthalene rings in rosette 26
compared to rosette 16, thus imparting these rosettes with
different morphological features.
changes are clear indications of the J-type stacking of the
aromatic chromophores.[10] In striking contrast, regioisomer 2
showed a hypsochromic shift (lmax = 442!405 nm) together
with a significant hypochromic effect upon cooling (blue line
in Figure 2b). The presence of red-shifted absorption
shoulder at 470 nm indicates a twisted stacking arrangement
of naphthalene chromophores.[11] The ei(300–550) value of 2
decreased from 251 to 176 Lmolꢀ1 cmꢀ1 upon going from 70 to
208C. These spectral changes are typical of the formation of
H-type stacking of aromatic molecules.
According to exciton theory,[12] J-type aggregation of dyes
open radiative decay pathway for the excited states, whereas
H-type aggregation results in nonradiative decay. Regioisom-
ers 1 and 2, when they are monomeric in tetrahydrofuran,
weakly fluoresce with structured emission bands (dotted lines
in Figure 2c,d). The low fluorescence quantum yields (FFL
ꢁ 0.0003) are attributed to nonradiative deactivation by
a bond-twisting in the excited states. On the other hand,
monomeric species in MCH at 708C became almost non-
emissive owing to enhanced thermal deactivation (red lines in
Figure 2c,d). With decreasing temperature, however, a prom-
inent emission was observed for 1 at 550 nm (Figure 2c). The
fluorescence quantum yield measured for the 1 ꢀ 10ꢀ4 solution
increased to 0.08 at 208C. This was not the case for 2, which
remained non-emissive upon decreasing the temperature
(Figure 2d). Although the enhanced emission of 1 is partly
due to the suppression of bond-twisting by aggregation,[13]
contrastive emission properties of these regioisomers in the
aggregated state further exemplify the distinct stacking
arrangements of naphthalene chromophores.
The UV/Vis absorption spectra of methylcyclohexane
(MCH) solutions of 1 and 2 recorded at 708C exhibited
vibronic transitions characteristic of molecularly dissolved
p systems (red lines in Figure 2a,b). Upon cooling, the
Dynamic light scattering (DLS) measurements were
performed for MCH solutions of 1 and 2 at three concen-
trations: 5 ꢀ 10ꢀ5, 1 ꢀ 10ꢀ4, and 5 ꢀ 10ꢀ4 m. For 1, average
hydrodynamic diameters (DH) of ca. 29 nm were always
observed upon changing the concentration (Figure 2e). The
concentration-independent DH is a clear indication of the
formation of stable closed aggregates. On the contrary,
regioisomeric 2 displayed DH of 310 nm already at 5 ꢀ 10ꢀ5
m
(Figure 2 f), which is one order of magnitude larger than that
of 1. More importantly, 2 showed a concentration-induced
increase of DH to 410 nm at 1 ꢀ 10ꢀ4 m and to 540 nm at 5 ꢀ
10ꢀ4 m. This observation suggests the formation of open-ended
aggregates.
Figure 2. Temperature-dependent a,b) UV/vis spectra and c,d) fluores-
cence spectra of a,c) 1 and b,d) 2 at a concentration of 1ꢀ10ꢀ4 m. Left
axes in (a,b) are shown with molar extinction coefficient e in
We used atomic force microscopy (AFM) to unveil the
dramatically different nanostructures of the aggregates. Fig-
ure 3a is the AFM image of J-aggregated 1 spin-coated from
a MCH solution onto highly oriented pyrolytic graphite
(HOPG). Perfectly toroidal nanostructures (nanorings) were
observed (Supporting Information, Figure S1). These nanor-
ings have remarkable uniformity in their height and size: the
height (hring) is (2.0 ꢂ 0.1) nm, and the top-to-top diameter
(d1ring) is (14 ꢂ 0.1) nm (Figure 3c). The edge-to-edge diam-
eter (d2ring) after accounting for the tip broadening effect is
(20 ꢂ 0.1) nm. Any other nanostructures could be scarcely
found by repeated AFM imaging in different areas. Remark-
ably, nanorings could be exclusively imaged for samples
prepared from solutions with a wide concentration range
between 5 ꢀ 10ꢀ5 m and 5 ꢀ 10ꢀ3 m. This situation is notably
different from our previous BAR–p-wedge assemblies, for
104 Lmolꢀ1 cmꢀ1. The spectra were recorded upon cooling from 708C
(red) to 208C (blue) at 108C intervals. The arrows indicate changes
upon cooling. Insets in (c,d) are photographs of the solutions at 208C
under UV light illumination. The green dotted spectrum in (a) is
a fluorescence excitation spectrum of 1 monitored at 550 nm. e,f) Dy-
namic light scattering of e) 1 and f) 2 in MCH at concentrations of
5ꢀ10ꢀ5 m (red), 1ꢀ10ꢀ4 m (green), and 5ꢀ10ꢀ4 m (blue).
formation of different types of aggregates was revealed for
these regioisomers by distinct spectral changes. Regioisomer
1 displayed a pronounced growth of bathochromically shifted
absorption bands (blue line in Figure 2a). The spectrum
recorded at 208C showed an increase of the integrated molar
absorptivity in the range of 300–550 nm (ei(300–550)) from
380 Lmolꢀ1 cmꢀ1 at 708C to 442 Lmolꢀ1 cmꢀ1. Such spectral
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with the top-to-top distance estimated for the densely packed
nanorods.
The foregoing nanostructures were further corroborated
by transmission electron microscopy (TEM). For nanorings of
1 (Figure 3e), their edge-to-edge diameters (d2ring) in the
TEM images diverged in the range between 18 and 27 nm,
which might be due to the deformation by vacuum evapo-
ration. For 2, both isolated and bundled nanorods could be
seen (Figure 3 f). Thus, the two regioisomers undergo specific
organization process in solution where J-type and H-type p–p
stacking of their naphthalene cores plays a key role to
determine the nanostructures.
To obtain insight into the mechanism underlying the
contrastive organization process and optical properties of the
regioisomers in solution, their organization in the bulk state
was investigated. Differential scanning calorimetry (DSC)
and polarized optical microscopy (POM) revealed that both
the compounds form thermotropic liquid crystalline meso-
phases in different temperature ranges (Figure 4a,b).[15]
Figure 3. a,b) AFM height images of a) 1 and b) 2 spin-coated from
MCH solutions (1: c=5ꢀ10ꢀ4 m; 2: c=1ꢀ10ꢀ4 m) onto HOPG.
c,d) Cross-sectional analysis along the white lines in (a) and (b),
respectively, with a representation of the nanostructures. e,f) TEM
images of e) 1 (scale bars=20 nm) and b) 2. The red and blue arrows
in (f) denote the isolated and bundled nanorods, respectively.
which increasing concentration resulted in the formation of
open-ended nanofibers.[7a,c]
In strong contrast, AFM images of the H-aggregates of 2
displayed monolayers of densely packed rodlike nanostruc-
tures (nanorods). The monolayers consist of domains of 3–10
oriented nanorods (Figure 3b; Supporting Information, Fig-
ure S2). The heights of nanorods (hrod) are uniform at (4.6 ꢂ
0.1) nm, and the average top-to-top distance (wrod) between
neighboring rods is 5.6 nm (Figure 3d). Taking into account
the deformation of nanorods by the adsorption to the
substrate and the local tip force, the similarity in these
dimensions suggests that the formation of cylindrical nano-
rods with the circular cross-section. For samples prepared
from more dilute solutions (ca. 5 ꢀ 10ꢀ5 m), isolated nanorods
could be imaged (Supporting Information, Figure S3), whose
edge-to-edge width reaches 19.2 nm. For such isolated nano-
structures, the tip-broadening effect is pronounced and
calculated to be 13.3 nm according to the reported
method.[14] The actual diameter of the isolated nanorods
thus can be estimated as 5.9 nm, which is in good agreement
Figure 4. a,b) Thermal phase-transition behavior of a) 1 and b) 2
studied by DSC; transition temperatures are shown, with enthalpies in
parentheses. c,d) Polarized optical micrographs and e,f) X-ray diffrac-
tion patterns of c,e) 1 in the Colr phase and d,f) 2 in the Colh phase.
For 1, POM texture of the Colt phase observed at a higher temperature
is shown as an inset in image (c).
Regioisomer 1 showed two mesomorphic states with the
boundary at 1478C. POM observation upon cooling from the
isotropic phase first exhibited a fan-shape texture typical of
columnar liquid crystals, which upon further cooling turned to
be a palmleaf-like texture at around 1478C (Figure 4c). The
X-ray diffraction (XRD) pattern of the higher-temperature
mesophase showed the formation of tetragonal columnar
structure (Colt) with the lattice constant a = 40.7 ꢁ (Support-
ing Information, Figure S4). On the other hand, the lower-
temperature mesophase exhibited two explicit diffractions at
d = 40.3 and 31.8 ꢁ, diagnostic of a rectangular columnar
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(Colr) ordering (Figure 4e). The presence of minor higher-
order diffractions allows the assignment of this phase as
a c2mm centered rectangular columnar (Colr) structure with
lattice constant a = 63.6 and b = 52.1 ꢁ.
Regioisomer 2 formed a mesophase in a very narrow
temperature range (Figure 4b), and exhibited high-aspect-
ratio linear defects upon cooling from the isotropic liquid
(Figure 4d). XRD measurement of this mesophase displayed
an intense diffraction peak at d = 46.0 ꢁ with a lot of minor
peaks that were perfectly assignable to diffractions from
a hexagonal columnar structure with the lattice constant a =
53.0 ꢁ (Figure 3 f). The lattice constant is in good agreement
with the width of nanorods estimated by AFM. The absence
of focal-conic defect may reflect the formation of an excep-
tionally stiff columnar assemblies in the bulk state.[16]
By using synchrotron radiation, we could observed the
intracolumnar periodicity of 3.6 ꢁ (lattice constant c or
center-to-center distance of stacked disks) for the Colh
mesophase of 2 (Supporting Information, Figure S5). This
periodicity allows the calculation of the number of molecules
in a columnar slice of 3.6 ꢁ thickness (Z value).[17] Assuming
the density of 2 to be 1.0 gcmꢀ3 from structurally related
compounds,[18] the Z value is calculated to be 6.0, which
strongly supports our previous hypothesis that BAR–p-wedge
molecules form hexameric hydrogen-bonded rosettes.[7] On
the other hand, the Colr mesophase of 1 showed no
diffractions that could be assigned to the intracolumnar
periodicity. We thus calculated its lattice constant c by giving
Z = 6 for a columnar slice of c ꢁ thickness. The calculated
lattice constant c is 5.3 ꢁ, which is notably longer than that of
the Colh phase of 2. The longer intracolumnar periodicity is
typical of rectangular columnar organization of discotics,
which is generally caused by a translational displacement to
form columns with ellipsoidal cross-section.[17,19]
The above XRD study clearly shows that the stacking
arrangements of the rosettes play a crucial role in their
hierarchical organization process. Molecular models of
rosettes 16 and 26 are shown in Figure 5a. The two rosettes
displayed a clear geometrical difference arising from the
conformation of naphthalene moieties. Rosette 16 shows
a relatively flat geometry because six naphthalene rings do
not rotate significantly owing to the absence of steric
repulsion with neighboring molecules. For such a planar
discotic entity with coplanar naphthalene moieties with
respect to the rosette plane, a lateral displacement would
occur upon stacking to minimize repulsive interaction
between p surface,[20] which is consistent with the Colr packing
in the mesophase. In aliphatic solvents, such a columnar stack
of rosettes covered by aliphatic wedges is well-solvated and
isolated, and lateral and most likely rotational displacement
between rosettes engender a spontaneous curvature of
columns, thus leading to the formation of nanorings (Fig-
ure 5b). It can be easily seen from simple geometrical
considerations that neither translational nor rotational dis-
placement of the rosette could give rise to an identical J-type
stacking arrangement for all the six naphthalene moieties
within a rosette. This situation is consistent with a rather
broad absorption band of fully aggregated 1 in MCH (Fig-
ure 2a); the remaining absorption around 400 nm indicates
Figure 5. a) Geometry-optimized (MMFF) structures of hexameric
rosettes of molecular modeled hexameric rosettes of 1 and 2.
b,c) Representation of the hierarchical organization of rosettes b) 16
and c) 26.
the existence of excitonically decoupled naphthalene chro-
mophores. The fluorescence excitation spectrum of fully
aggregated 1 monitored at 550 nm indeed showed a low
contribution of these higher-energy bands to the green
emission (green dotted curve in Figure 2a).
In contrast, rosette 26 shows a crown-gear-like morphol-
ogy owing to the rotation of six naphthalene rings to cancel
the steric repulsion with neighboring molecules. For the
stacking of such discotics, lateral displacement should not be
allowed owing to steric hindrance induced by out-of-plane
naphthalene moieties. Instead, a rotational displacement of
the rosette would allow an optimal stacking arrangement[21]
whereby out-of-plane naphthalene moieties can take a H-type
p–p stacking interactions with a local rotational displacement,
as seen by the red-shifted absorption shoulder (Figure 2b).
The extension of this stacking model leads to the formation of
a cylindrical rod featuring helical H-aggregates of naphtha-
lene chromophores (Figure 5c).[22]
To obtain insight into the optoelectronic properties of the
above nanostructures, we performed flash-photolysis time-
resolved microwave conductivity (FP-TRMC) experiments,
which enable electrodeless evaluation of photogenerated
mobile charge carriers over a short distance (about 10 nm).[23]
Upon excitation with a 355 nm laser pulse, the nanorings of
1 showed the maximum transient conductivity fꢀmmax of 1.1 ꢀ
10ꢀ5 cm2 Vꢀ1 sꢀ1 (where f= quantum yield of photocarrier
generation and ꢀm = sum of the mobilities of photogenerated
charge carriers; Figure S6). Remarkably, the nanorods of 2
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exhibited a value fꢀmmax of 4.7 ꢀ 10ꢀ5 cm2 Vꢀ1 sꢀ1, which is
more than four times higher than that of the nanorings. The
significant difference observed between the transient con-
ductivities of the two contrasting nanoscopic aggregates
illustrates that the nanorods can act as a superior semi-
conducting nanomaterial compared to the nanorings owing to
the larger overlap (H-type stacking) as well as uniform p–p
stacking arrangements of the naphthalene p systems.[24]
In conclusion, we have demonstrated that the regioiso-
meric introduction of barbituric acid and an aliphatic wedge
into the naphthalene p system enables perfect control over
the self-assembled nanostructures of our BAR–p-wedge
molecules. The formation of distinct 0D and 1D nanostruc-
tures (nanorings and nanorods) by the regioisomers is
ascribable to offset and face-to-face stacking arrangements
of hydrogen-bonded cyclic assemblies (rosettes). This is, in
turn, attributed to the conformational difference of naphtha-
lene moieties with respect to the plane of rosettes. Of
particular interest is that such different stacking modes of
the rosettes also induced the representative stacking arrange-
ments of naphthalene chromophores, as revealed by their
characteristic optical and optoelectronic properties. Such
simultaneous control over stacking fashion and self-assem-
bled nanostructures for functional p assemblies is one of the
most challenging goals in this research field toward nano-
structured materials with tailor-made dimensionality and
functionality. The self-sorting study of the regioisomers and
extension of the p systems in the BAR–p-wedge design are
underway.
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Received: February 21, 2012
Revised: April 5, 2012
Published online: May 24, 2012
Keywords: H-aggregates · J-aggregates · nanorings ·
.
self-assembly · supramolecular chemistry
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